1. Field of the Invention
The present invention relates to a descending grade condition detecting apparatus which can detect a descending grade such as a snow-packed road or the like where slippage is likely, and to a brake controlling apparatus which can perform anti-skid control and the like based on a descending grade condition detected by this descending grade condition detecting apparatus.
2. Description of Related Art
In recent years, increased implementation of electronic control relating to the suspension of a vehicle has been remarkable, and an anti-skid control apparatus is known as a representative example thereof.
According to this anti-skid control apparatus, as shown in FIG. 17, estimated vehicle body speed is calculated based on wheel speed obtained by wheel speed sensors arranged at the vehicle wheels, and estimated vehicle body deceleration indicating change in estimated vehicle body speed (i.e., indicating a slope of estimated vehicle body speed) is calculated from estimated vehicle body speed. Accordingly, the estimated vehicle body deceleration calculated in this way may be employed as road surface .mu. information, and this estimated vehicle body deceleration may be used as a control parameter for anti-skid control.
That is, when anti-skid control is performed, estimated vehicle body deceleration is sequentially calculated at each unit of time, control of brake hydraulic pressure is performed in accordance with the values of estimated vehicle body deceleration and hydraulic control corresponding to the road surface .mu. is implemented.
However, in a vehicle equipped with an anti-skid control apparatus such as this, when braking has been performed on a low .mu. descending grade which is susceptible to slippage, such as a snow-packed road or the like, the wheels may rotate in excess of what is predicted due to an effect of low hydraulic pressure for control by anti-skid control and of gravitational forces which act in the gradient direction of the descending grade, and because of this, a decline in estimated vehicle body deceleration formulated based on wheel speed may occur.
That is, when braking has been performed on a descending grade which is susceptible to slippage, such as a snow-packed road or the like, wheel speed becomes greater in comparison with a case where braking has been performed on a level road having a similar road surface .mu., and as a result thereof, estimated vehicle body speed becomes large, as shown by the dotted line in FIG. 17. For this reason, the estimated vehicle body deceleration (i.e., the slope of the graph) calculated from this estimated vehicle body speed becomes small.
That is, because estimated vehicle body deceleration becomes smaller on a descending grade than on a level road, even when the descending grade and level road are similar snow-packed roads, an extent of slippage is considered to be larger (being an extremely low .mu. road of, for example, an icy road) on the descending grade even with similar brake hydraulic pressure, and to alleviate slippage, control is executed on the side to lower brake hydraulic pressure. As a result thereof, there exists a problem wherein braking performance is reduced on a descending grade in comparison with a case where braking is performed on a level road.